Solar cells are currently utilized as an energy source by using their ability to convert sunlight to electrical energy. Silicon is used almost exclusively as the semiconductor material in such photovoltaic cells. A significant limitation currently on the use of solar cells has to do with the cost of purifying silicon to solar grade (SG). In view of current energy demands and supply limitations, there is an enormous need for a more cost efficient way of purifying metallurgical grade (MG) silicon (or any other silicon having greater impurities than solar grade) to solar grade silicon.
In a process to purify a material using a metallic solvent, valuable material is left in the metallic solvent along with the impurities. For example, in silicon purifying processes using a solvent metal, valuable silicon is left in the by-products. Repeated attempts to fractionally crystallize the silicon result in a proportionally increasing loss of silicon in the by-products. Companies and research groups have been working on making upgraded metallurgical (UMG) silicon using metallurgical processes involving various hydrometallurgical and pyrometallurgical processes. Many of these processes are limited in that they have difficulty reducing the amount of phosphorous relative to that of boron. Ultimately, this results in a final material that has an excess of phosphorus. The amount of phosphorous is higher than the amount of boron in ppmw in the UMG silicon after purification and the amount of phosphorous is usually 2-3 times higher than the boron in ppmw.
Traditionally, the majority of solar cells are made using P-type semiconductors with higher boron levels than phosphorous levels. The UMG silicon is directionally solidified into boules or multicrystalline ingots. This directional solidification of UMG silicon increases the phosphorous level in the top of the ingot due to the difference of the segregation coefficients of boron and phosphorous. This concentration transition can cause a P/N transition to form between the areas of high phosphorus concentration and the areas of lower phosphorus concentration, reducing yield and also causing the resistivity to change with the height of the ingot. Most solar cells not made from UMG silicon are made by adding P-type boron-containing dopants to silicon purified by the Siemens process.